The present invention relates to a method and apparatus for fabricating a three-dimensional element from an anisotropic material, and more particularly to a method and apparatus for fabricating a three-dimensional element from a thin film or monocrystal of high-temperature superconductive material by means of focused-ion-beam machining performed in accordance with a special fabrication method.
A conventional focused-ion-beam etching apparatus can assume an inclined orientation at an angle as large as 60xc2x0. Further, machining an object from a side surface thereof has not been attempted.
Conventionally, in order to obtain an intrinsic Josephson effect, supercurrent must be caused to flow through a stack of monocrystals of a high-temperature superconductor having a layer structure (herein after simply called as a xe2x80x9clayered high-temperature superconductorxe2x80x9d), in the stack direction. Further, in order to obtain a practical device, the length of a path for supercurrent must be adjusted with an accuracy corresponding to the size of crystals.
FIG. 1 is a schematic cross section of such a conventional electronic element.
In FIG. 1, reference numeral 101 denotes a stack of high-temperature superconductive monocrystals; reference numeral 102 denotes a projection formed by the stack of superconductive monocrystals; reference numeral 103 denotes an insulating layer formed on the surface of the stack of superconductive monocrystals 101 excluding the projection 102; and reference numeral 104 denotes an electrode connected to the projection 102.
Such a conventional electronic element is fabricated in such a manner that the fine projection 102 is formed on the top surface of the stack of superconductive monocrystals 101 by use of a chemical or physical etching technique, and is used in a state in which supercurrent is caused to flow through the stack of superconductive monocrystals 101 in the stack direction.
Conventionally, in order to obtain a single-electron tunnel element, a tunnel junction layer must be formed with sub-picometer accuracy, in order to decrease its electrostatic capacitance. Therefore, the reproducibility of the element is poor. Further, the element operates at very low temperature (1 K or lower) only.
Moreover, in the conventional method of fabricating an electronic element, the element is formed through machining performed from the top surface of a monocrystal or thin film. Therefore, the uniformity of the surface has been important.
Furthermore, a conventional mesa-type electronic element for obtaining the intrinsic Josephson effect cannot be formed on a substrate having a hole.
As described above, the conventional process for fabricating an electronic element involves the following problems.
(1) A conventional intrinsic Josephson device cannot be fabricated by use of a c-axis-oriented thin film formed of a layered high-temperature superconductor of high quality, because the length of a supercurrent path cannot be controlled accurately.
(2) A single-electron tunnel device requires machining within a very small area whose sides are shorter than one picometer, thereby rendering reproducibility poor. Further, fabrication of a device utilizing a stack of c-axis-oriented thin films of high quality has been impossible.
(3) Since a conventional focused-ion-beam etching apparatus can assume an inclined orientation at an angle as large as 60xc2x0, a superconductive thin film on a substrate cannot be etched from a side surface thereof.
(4) When a monocrystal is used, a conventional process requires complicated steps, such as both-face machining and reversing of a sample, and many samples break in these steps. Further, since machining is performed from the top face of a substrate, machining accuracy in the depth direction is affected by surface uniformity.
An object of the present invention is to solve the above-described problems and to provide a method and apparatus for fabricating a three-dimensional element from an anisotropic material. Examples of the three-dimensional element include a single-electron tunnel device and an intrinsic Josephson device which utilize a layer structure peculiar to a layered high-temperature superconductor. The three-dimensional element can be fabricated from a stack of c-axis-oriented thin films formed of a layered high-temperature superconductor of high quality, and the length of a tunnel junction can be controlled accurately through measurement of an image displayed on a screen. Further, the fabrication of the three-dimensional element does not require a step of reversing a sample during in-situ machining, and the three-dimensional element can be fabricated through fine area machining from the side surface of a monocrystal or thin film, without being affected by the surface uniformity of the monocrystal or thin film.
To achieve the above object, the present invention provides the following:
[1] A method for fabricating a three-dimensional element by use of an anisotropic material, characterized by comprising the steps of:
forming a thin film having anisotropy and a bridge on a substrate for thin-film growth; and
mounting the substrate onto a sample holder, rotating the sample holder to an angle of 360xc2x0, and machining the bridge from the side surface thereof by means of focused-ion-beam machining.
[2] A method for fabricating a three-dimensional element by use of an anisotropic material, characterized by comprising the steps of:
forming a monocrystal having anisotropy and a bridge on a substrate for attachment of the monocrystal; and
mounting the substrate onto a sample holder, rotating the sample holder to an angle of 360xc2x0, and machining the bridge from the side surface thereof by means of focused-ion-beam machining.
[3] A method for fabricating a three-dimensional element by use of an anisotropic material described in [1] or [2] above, wherein the bridge is machined three-dimensionally by means of focused-ion-beam machining to thereby obtain a single-electron tunnel junction device which is formed of a layered high-temperature superconductor and which has a tunnel junction layer of a very small area.
[4] A method for fabricating a three-dimensional element by use of an anisotropic material described in [1] or [2] above, wherein the bridge is machined three-dimensionally by means of focused-ion-beam machining to thereby obtain an intrinsic Josephson junction device which is formed of a layered high-temperature superconductor and which has a supercurrent path layer of a very small area.
[5] A method for fabricating a three-dimensional element by use of an anisotropic material described in [3] above, wherein the layered high-temperature superconductor is a c-axis-oriented thin film of a layered high-temperature superconductor, and the single-electron tunnel junction device is a single-electron tunnel junction device which has a tunnel junction layer of a very small area not greater than one square micrometer and utilizes an intrinsic layered structure.
[6] A method for fabricating a three-dimensional element by use of an anisotropic material described in [4] above, wherein the layered high-temperature superconductor is a c-axis-oriented thin film of a layered high-temperature superconductor, and the intrinsic Josephson junction device is an intrinsic Josephson junction device which has a supercurrent path layer of a very small area not greater than one square micrometer and utilizes an intrinsic layered structure.
[7] A method for fabricating a three-dimensional element by use of an anisotropic material described in [3] above, wherein the layered high-temperature superconductor is a c-axis-oriented monocrystal of a layered high-temperature superconductor, and the single-electron tunnel junction device is a single-electron-pair tunnel junction device which has a tunnel junction layer of a very small area not greater than one square micrometer and utilizes an intrinsic layered structure.
[8] A method for fabricating a three-dimensional element by use of an anisotropic material described in [4] above, wherein the layered high-temperature superconductor is a c-axis-oriented monocrystal of a layered high-temperature superconductor, and the intrinsic Josephson junction device is an intrinsic Josephson junction device which has a supercurrent path layer of a very small area not greater than one square micrometer and utilizes an intrinsic layered structure.
[9] A method for fabricating a three-dimensional element by use of an anisotropic material described in [2] above, wherein an intrinsic Josephson junction device using a c-axis-oriented monocrystal of a layered high-temperature superconductor is formed on a substrate having a hole by means of focused-ion-beam machining.
[10] An apparatus for fabricating a three-dimensional element by use of an anisotropic material, characterized by comprising a sample stage which is disposed such that a sufficient clearance is formed between the sample stage and peripheral equipment and which can effect 6-axis motion including a tilting motion from 0xc2x0 to 360xc2x0 in both positive and negative directions.
[11] An apparatus for fabricating a three-dimensional element from an anisotropic material, characterized by comprising a sample stage which is equipped with a jig having a slant angle of 30xc2x0 to 90xc2x0.